Francesco Di Natale

Cadmium adsorption by coal combustion ashes-based sorbents—Relationship between sorbent properties and adsorption capacity

A very interesting possibility of coal combustion ashes reutilization is their use as adsorbent materials, that can also take advantage from proper beneficiation techniques. In this work, adsorption of cadmium from aqueous solutions was taken into consideration, with the emphasis on the intertwining among waste properties, beneficiation treatments, properties of the beneficiated materials and adsorption capacity. The characterization of three solid materials used as cadmium sorbents (as-received ash, ash sieved through a 25 μm-size sieve and demineralized ash) was carried out by chemical analysis, infrared spectroscopy, laser granulometry and mercury porosimetry. Cadmium adsorption thermodynamic and kinetic tests were conducted at room temperature, and test solutions were analyzed by atomic absorption spectrophotometry. Maximum specific adsorption capacities resulted in the range 0.5-4.3 mg g(-1). Different existing models were critically considered to find out an interpretation of the controlling mechanism for adsorption kinetics. In particular, it was observed that for lower surface coverage the adsorption rate is governed by a linear driving force while, once surface coverage becomes significant, mechanisms such as the intraparticle micropore diffusion may come into play. Moreover, it was shown that both external fluid-to-particle mass transfer and macropore diffusion hardly affect the adsorption process, which was instead regulated by intraparticle micropore diffusion: characteristic times for this process ranged from 4.1 to 6.1d, and were fully consistent with the experimentally observed equilibrium times. Results were discussed in terms of the relationship among properties of beneficiated materials and cadmium adsorption capacity. Results shed light on interesting correlations among solid properties, cadmium capture rate and maximum cadmium uptake.

Reuse of Coal Combustion Ash as Sorbent: The Effect of Gasification Treatments

Beneficiation through gasification of coal combustion ash coming from an industrial power plant aimed at its reuse as adsorbent is addressed. The raw ash was gasified in a tubular reactor at 850°C for times ranging from 10 to 60 min, using either steam or CO2 as gasifying agents. The relationships among ash properties, gasification treatments, and properties of the gasified ashes deserve investigation. Therefore, ash characteristics (such as carbon content and porosimetric, diffractometric, and granulometric properties) were correspondingly determined. In the light of a critical analysis of the obtained results, gasification conditions able to make the parent ash a potentially better adsorbent material were defined, with a particular emphasis on similarities and differences between steam and CO2 gasification.Finally, preliminary results concerning cadmium adsorption from aqueous solutions have been reported.

Colloidal Carbon-Based Nanoparticles as Heavy Metal Adsorbent in Aqueous Solution: Cadmium Removal as a Case Study

Hydrophilic carbonaceous nanoparticles (HNPs) of uniform sizes with a good degree of dispersion in water were produced from a commercial carbon black by nitric acid treatment. The surface treatment, performed at different reaction times, generates a variable number of oxygen functional groups, mainly carboxylic, which enhance the nanoparticles hydrophilicity and heavy metal adsorption capability. The HNPs were characterized by a number of analytical techniques, including FTIR spectroscopy, thermal and elemental analysis, N2 adsorption, dynamic light scattering, and zeta-potential measurements. The acid–base properties of the functional groups on the HNPs surface were also investigated by coulometric–potentiometric titrations. Cadmium adsorption tests were carried out in stirred reactors containing colloidal aqueous suspensions of HNPs and HNPs supported over silica. The effects of several parameters, such as the cadmium concentration, the temperature, and the solution pH, were studied. Sorbents showed an appreciable cadmium adsorption capability at different temperatures and in a wide range of pH values comparable or superior to several carbon-based sorbents, indicating a feasible use in commercial units.

Enhanced So2 Removal by Using Charged Water Droplets

This paper reports preliminary results on the absorption of SO2 from water charged droplets, produced by an electrospray operated in dripping mode. Our results show that the water charging allows a ≈50% increase of the SO2 absorption rate. Since the SO2 mass transfer coefficient depends mainly on the liquid-side mass transfer coefficient, we envisage that the increase of SO2 absorption rate is ascribable to the occurrence of droplet deformation and oscillation, driven by the reduction of surface tension caused by the presence of excess charge on the droplet surface. Experiments were successfully interpreted in light of an absorption model that specifically address the droplet formation and fall and considered the model of Matteson and Giardina (1974) to account the droplet charge effect.

Local heat transfer coefficients and superficial bed porosity of a horizontal cylinder in bubbling fluidized beds of geldart B particles

In this work, experimental values of local heat transfer coefficients around a horizontal cylinder immersed in a bubbling fluidized bed are reported for three types of bed materials classified as Geldart B particles, fluidized with air at ambient pressure and temperature. Results are interpreted in light of a model for heat transfer coefficient in order to estimate the time-average bed porosity profile close to the exchange surface. These angular profiles of bed porosity are compared with former experiments to verify the correctness of the adopted model, and are used to provide a physical interpretation of the experimental results.

DBD plasma for NOx adsorption and desorption-reduction using GAC for the marine emissions control

Abatement of NO using adsorption, desorption and reduction (ADR) using Dielectric Barrier Discharge (DBD) is presented. The NO was adsorbed on a coconut shell granular activated carbon (GAC) impregnated with copper (2%). The ADR setup consisted in a sequence of DBD reactors. The first one also contained the GAC. The adsorption was carried out at about 32°C with a gas mixture of 250 ppm of NO in air (10% of O2) at the gas velocity of 0.1 m/s. The mechanisms of NO uptake include simple adsorption and catalytic reduction on the copper impregnated GAC. During the desorption/reduction process, a stream of N2 at the gas velocity of 0.1 m/s was passed through the reactors in presence of non-thermal plasma. A set of voltages between 10 to 36 kVpk_pk and the frequency of 800–1400 Hz was tested to obtain the required plasma for an efficient NO desorption and reduction. The experimental results showed that the ADR system is able to reduce >96% of the NO with a single reactor and >99% of NO with two reactors in series. Besides, cyclic ADR tests indicated that the GAC preserve more than 90% of its uptake capacity after 30 cycles.